The present invention relates to the techniques of data retransmission in telecommunications systems. It relates more particularly to the control of data retransmission in systems using a data acknowledgement mode.
Numerous telecommunications systems use a data acknowledgement mode which, for a communication object receiving data, consists in an acknowledgement of reception, the transmission of which is initiated by the receiver or requested by the transmitting object. Conversely, if the receiving object does not receive or receives incorrectly certain data which were addressed to it, it can send to the transmitting object a negative acknowledgement for these data, thus indicating the failure of the transmission of the corresponding data.
Conventionally, a data retransmission mechanism can be based on the acknowledgement mechanism. In particular, when a negative acknowledgement is received by the data transmitting object, the latter can decide to retransmit the data to which this negative acknowledgement related.
An example of a retransmission mechanism of this type is that specified in third-generation cellular networks of the UMTS (Universal Mobile Telecommunication System) type, standardized by the 3GPP (3rd Generation Partnership Project) organization.
The invention is described below, without intent to limit the generality of its object, in its application to a UMTS network in frequency division duplex. (FDD) mode, an example of the architecture of such a network being shown in FIG. 1.
The switches of the mobile service 10, belonging to a core network (CN), are connected, on the one hand, to one or more fixed networks 11, and, on the other hand, to control units or RNCs (Radio Network Controllers) 12 by means of what are known as Iu interfaces. Each RNC 12 is connected to one or more radio base stations 13 by means of what are known as Iub interfaces. The radio stations 13, distributed over the network coverage area, can communicate by radio with the mobile terminals 14, 14a, 14b called UE (User Equipment). The radio stations can be grouped to form nodes, called “Nodes B”. Some RNCs 12 can also communicate with each other by means of what is known as an Iur interface. The RNCs and the radio stations form an access network called UTRAN (UMTS Terrestrial Radio Access Network).
The UTRAN comprises elements of layers 1 and 2 of the ISO (International Standard Organization) model in order to provide the required links over the radio interface (called Uu), and a radio resource control (RRC) stage 15A belonging to layer 3, as described in technical specification 3G TS 25.301, “Radio Interface Protocol Architecture”, version 4.2.0, published in December 2001 by 3GPP. Viewed from the higher layers, the UTRAN acts simply as a link between the UE and the CN.
FIG. 2 shows the RRC stages 15A, 15B and the stages of the lower layers which belong to the UTRAN and to a UE. On each side, layer 2 is divided into a radio link control (RLC) stage 16A, 16B and a medium access control (MAC) stage 17A, 17B. A detailed description of the radio link control can be found, in particular, in technical specification TS 25.322, version 5.1.0, “Radio Link Control (RLC) protocol specification”., published by 3GPP in June 2002. Layer 1 comprises an encoding and multiplexing stage 18A, 18B. A radio stage 19A, 19B transmits the radio signals on the basis of trains of symbols supplied by the stage 18A, 18B, and receives the signals in the other direction.
There are different ways of adapting the protocol architecture according to FIG. 2 to the hardware architecture of the UTRAN according to FIG. 1, and, in general, different organizations can be adopted according to the types of channel (see section 11.2 of technical specification 3G TS 25.401 “UTRAN Overall Description”, version 4.2.0, published in September 2001 by 3GPP). The RRC, RLC and MAC stages are located in the RNC 12. Layer 1 is located, for example, in Node B. Part of this layer may, however, be located in the RNC 12.
In one mode of operation of the system, the RLC frames are exchanged in acknowledged mode. Thus, a polling bit can be activated in certain RLC frames sent by an RNC 12 to a UE, in order to interrogate the UE about the reception of one or more RLC data frames transmitted previously. The UE responds to the polling with an RLC signal frame containing an acknowledgement, which may be positive (ACK) or negative (NACK), and which indicates the correct or incorrect reception of the RLC data frames transmitted previously. Various algorithms can be used by the RLC layer 16A of the RNC to process the positive and/or negative acknowledgements transmitted by the UE. In general, the RNC 12 retransmits the RLC data frames to which the NACK related, on receiving the latter.
When a plurality of polling signals are sent in succession by the RNC 12, for example following the loss of a data frame j (according to a sequence number SN) sent to a UE (reference NOK in FIG. 3), the same number of successive NACKs are sent by the UE. If the frame j is retransmitted following the reception of a NACK at the RNC, it is possible, on this occasion, that the corresponding data may be received correctly at the UE (reference OK in the figure). If this retransmission takes place immediately after polling, for example, a NACK will still be sent by the UE, as shown in FIG. 3. Consequently, when the RNC receives the NACK, the data will have been successfully received at the UE following the last retransmission.
There are two principal known solutions for preventing a further retransmission from taking place when the frame j of the preceding example has been correctly received after an initial transmission failure.
According to a first method, the UE triggers a timeout, called Timer_Status_Prohibit, described in section 9.5 of the technical specification 25.322 cited above, on receiving the first polling that it receives. The next polling signals received are ignored by the UE until the expiry of the timeout for preventing the sending of a second NACK to the UE when it is possible for the retransmission of the data following the sending of the first NACK by the UE to result in correct reception at the UE. Thus, if the retransmitted data have actually been received before the expiry of the timeout, the next acknowledgement sent by the UE can be an ACK, making it possible to avoid further unnecessary retransmissions.
According to a second method, the UE sends a NACK in response to each polling without distinction, following non-reception or incorrect reception of data, but the NACKs are filtered at the RNC 12. Thus, the second NACK transmitted by the UE in the example shown in FIG. 3 is ignored by the RNC if too short a time has elapsed since the first NACK was received by the RNC. This period is conventionally determined by a timeout, as in the preceding case, but is monitored at the RNC. Conversely, if a new NACK is received at the RNC after the expiry of the timeout, this NACK is responded to, resulting in a further retransmission, since it may indicate that the first retransmission has not resulted in correct reception at the UE.
In the two methods outlined above, the timeouts considered should ideally have the value of the round trip delay (RTD) between the RNC and the UE. This is because the time of an RTD is required for the UE to receive the data retransmitted after the sending of a NACK (first method), and for the RNC to receive a significant acknowledgement following the transmission of data (second method). Conversely, a timeout longer than the RTD could cause a delay in taking into account the positive acknowledgements at the RNC, thus slowing down the useful transmission speed, particularly in cases where the quantity of data which can be transmitted by the RNC without waiting for the acknowledgement of the previously transmitted data is small.
The estimation of the RTD poses a significant problem, since this value is variable. It can also undergo major variations if the data have to pass through an asynchronous communication interface.
This is the case, in particular, in the UMTS system, in the Iub interface between an RNC 12 and radio stations 13 using protocols such as ATM (Asynchronous Transfer Mode) and AAL2 (ATM Adaptation Layer No. 2). Above these protocol layers, a frame protocol (FP) is used in the user plane to enable the RNC to communicate with the Node B or nodes involved in a communication with a given UE. The FP is described in technical specifications 3G TS 25.427, “UTRAN Iub/Iur Interface User Plane Protocol for DCH Data Streams”, version 4.3.0, published in December 2001 by 3GPP.
When a plurality of RNCs are involved in a communication with a UE, there is generally a serving RNC. (SRNC), in which are located the modules relating to layer 2 (RLC and MAC), and at least one link RNC called the DRNC (Drift RNC) to which is connected a radio station with which the UE is in radio connection. In this case also, appropriate protocols such as ATM and AAL2 provide asynchronous exchanges between these RNCs through the Iur interface.
Additionally, UMTS in FDD mode supports a macrodiversity technique, in which one UE is enabled to communicate simultaneously with separate radio stations of an active set in such a way that, in the downlink direction, the UE receives the same information several times, and, in the uplink direction, the radio signal sent by the UE is captured by the radio stations to form different estimates which are subsequently combined in the UTRAN. Macrodiversity provides a gain in reception which improves the performance of the system, owing to the combination of different observations of a single item of information. It also makes it possible to provide “soft hand-off” (SHO) intercellular transfers when the UE moves.
In macrodiversity, the switching of the transport channels for multiple sending from the UTRAN or the UE and the combination of these transport channels in reception are operations carried out by a selection and combination module belonging to layer 1. This module is interfaced with the sub-layer MAC, and is located in the RNC serving the UE. If the radio stations concerned depend on different RNCs communicating through the Iur interface, one of these RNCs can act as the SRNC and the other as the DRNC.
Thus the value of the RTD between an RNC and a UE can vary considerably, particularly owing to the asynchronous nature of the Iub interface. If a radio station for which the Iub interface is slow (loaded) at the instant in question is added to or withdrawn from the active set, the RTD abruptly undergoes large variations because of the synchronization mechanisms implemented by the FP.
Because of its asynchronous nature and the fact that the routing times over all the links of the active set are taken into account, the Iub interface is largely responsible for the delays in transmission between an RNC 12 and a UE 14–14a–14b, by comparison, in particular, with the transmission delays over the Uu interface.
Another factor which can vary the RTD is the frame processing time at Node B, which can vary with time and from one Node B to another.
Even if the RTD can be estimated, the procedure for changing the Timer_Status_Prohibit parameter is only reactive to a small extent, since it requires a reconfiguration of the RLC connection. In practice, therefore, the first method above does not provide an effective means of adaptation to the variations of RTD.
An object of the present invention is to propose a good compromise between the number of retransmissions of data and the risks of decreasing the debit utile because an acknowledgement of the data is taken into account too slowly.
Another object of the invention is to enable the number of superfluous retransmissions in a communication system to be reduced without the need to estimate the RTD of the system. The invention is particularly applicable to cases where the communication system has an RTD varying significantly with time, for example because of the presence of an asynchronous communication interface.